This invention relates to coating systems. In one aspect, the invention relates to coating systems based on .alpha.,.beta.-ethylenically unsaturated oligomers while in another aspect, the invention relates to such systems in an emulsified form. In yet another aspect, the invention relates to such emulsified systems comprising a nonionic surfactant containing allyl unsaturation.
The increasing demands for coating systems with reduced volatile organic compounds (VOC) have brought radiation and peroxide curable coatings to the forefront. These systems can be formulated with little, if any, solvent, and they cure rapidly to form highly crosslinked polymeric structures.
The resins commonly used in radiation and peroxide curable coatings are low to medium molecular weight unsaturated polyesters, and acrylate and methacrylate esters of polyethers, polyesters, epoxies and polyurethanes. In order to render these relatively viscous oligomers suitable for application by conventional coatings techniques, they are diluted in low viscosity reactive monomers such as trimethylolpropane triacrylate, 1,6-hexanedioldiacrylate, N-vinyl-2-pyrrolidone, tetraethyleneglycol diacrylate, ethyleneglycol dimethacrylate, styrene, and the like.
Peroxide and radiation curable coatings suitable for roll coater or spray application usually contain 30-60 percent by weight of such reactive diluent monomers to achieve sufficiently low viscosities for application of a smooth film. However, since monomer-diluted coatings undergo, relative to solvent-diluted coatings, little volume reduction through evaporation, it is difficult to achieve very thin films with conventional application equipment, e.g. films with a thickness of less than about 0.5 mil.
Such large amounts of reactive monomer generally result in a reduction in a cure rate, decrease in physical properties of the cured film, unpleasant odor, and an increase in toxicity of the coating.
Many of the most effective monomers for reducing viscosity and maintaining good film integrity have the highest levels of toxicity. Higher molecular weight monomers with higher reactive group content have less effect on rate of cure and film properties, but do little for reducing viscosity.
The replacement of reactive monomer with water offers a route to lower viscosity, reduced odor, rapid cure and better film properties.
Water-soluble oligomers containing hydrophilic groups such as polyethylene glycol or neutralized acidic groups achieve some of the above-mentioned goals, but are inherently water sensitive in the cured film.
Conventional radiation or peroxide curable emulsions or dispersions are prepared with anionic, cationic or nonionic surfactants. These surfactants are nonreactive, nonvolatile components which lead to a reduced cure response, softer films and less resistance to water and organic solvents. In highly crosslinked films these surfactants tend to bleed out or "surface scum" during cure of the oligomer or upon aging.